Many organic compounds are known to be useful as photoconductive materials for electrophotographic light-sensitive compositions. Furthermore, some organic compounds have very high sensitivity. In practice, however, such organic photoconductive materials are rarely used in the preparation of electrophotographic materials.
Organic photoconductive materials have many advantages over inorganic photoconductive materials. Therefore, it is believed that they will have many uses in the field of electrophotography. Only organic photoconductive materials permit the production of a transparent electrophotographic light-sensitive film, a flexible electrophotographic light-sensitive film, an electrophotographic light-sensitive film which is light in weight and can be easily handled, and so forth. Furthermore, organic photoconductive materials have other excellent characteristics, such as their film-forming properties during the preparation of an electrophotographic light-sensitive material, smooth surface-forming properties, selectivity of charging polarity when used in an electrophotographic process, and forth. Inorganic photoconductive materials do not possess such properties.
However, mainly because of their low light sensitivity and brittleness when in the form of a coating organic photoconductive materials are hardly used in the field of electrophotography even though they have many excellent characteristics.
Investigation have been carried out with respect to heterocyclic compounds having low molecular weight, nitrogen-containing aromatic compounds, and various polymer type aromatic compounds to find organic photoconductive materials having high sensitivity. However, it has now been found that even those compounds which are known to be the most sensitive of organic photoconductive materials are not suitable for practical use without the application of a sensitization processing because their sensitivity is still insufficient. Accordingly, recent studies have been carried out to develop a sensitization method so that higher sensitivity can be obtained. In order to make practical use of organic photoconductive materials, it is essential to choose and apply the most effective sensitization method. The industrial value of an organic photoconductive material is determined to very large extent by the sensitization method being employed because the sensitization method is the biggest factor effecting the sensitivity of an electrophotographic light-sensitive material containing the organic photoconductive material.
The sensitization method which has been most well known in the art is a method involving the addition of a sensitizing dye or Lewis acid. This methed can be applied to almost all organic photoconductive materials. When adding a sensitizing dye, the spectral absorption characteristics of the dye are added to an organic photoconductive material. When adding a Lewis acid, a donor acceptor complex is formed between the Lewis acid and the organic photoconductive material, and this results in the appearance of new spectral sensitivity.